archive: SETI In Cosmic Blasts, Clues to Black Holes

SETI In Cosmic Blasts, Clues to Black Holes

Larry Klaes ( )
Wed, 26 May 1999 09:57:52 -0400

>Date: Tue, 25 May 1999 20:56:13 -0400
>Organization: SkyViews Astronomy & Space information Web Site
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>Subject: NEWS
>A gamma ray burst on May 10 was recorded at the European Southern
Observatory in Cerro Paranal, Chile, as an
> extra dot of light, apparently 10 billion light-years
distant, in the constellation Chamaeleon. The image at left was
> taken with the new Very Large Telescope shortly after the
burst was detected by satellites, the image at right 13
> years earlier.
> May 25, 1999
> In Cosmic Blasts, Clues to Black Holes
> Related Article
> Reports on Most Violent Events in Universe (March 26)
> Forum
> Join a Discussion on Science in the News
> BALTIMORE -- Frustrated by the mystery of the gamma ray
bursts, cryptic
> messages from what must be the most violent events in the
> astrophysicists have for the last 30 years had reason to
envy the certitude
> of Clarence Oddbody, George Bailey's guardian angel in the
evergreen movie "It's
> a Wonderful Life."
> Clarence had no trouble deciphering the signals of one
particular kind of
> transforming event in the heavens. "Every time you hear a bell
rings," he told
> George, "it means some angel's just got his wings."
> It may well be, astrophysicists now think, that the sudden,
random and faraway
> flashes of gamma radiation also are cosmic proclamations, but
of a secular kind.
> The enormous blasts of energy seem to be birth announcements
of another
> phenomenon of awesome power, one predicted by Einstein's
general theory of
> relativity but shrouded in mystery because no light can escape
the dark clutches
> of its consuming gravity.
> If current hypotheses are correct, Dr. Tsvi Piran of Columbia
University said at a
> four-day symposium at the Space Telescope Science Institute
here this month,
> "each gamma ray burst is signaling the creation of a new black
> Introducing one enigmatic phenomenon, black holes, to explain
another may not
> seem like progress. Definitive knowledge of black holes is
scant because they
> cannot be observed directly, though their existence and
behavior are inferred from
> the powerful effects they have on surrounding matter. And
about all astronomers
> have been able to say for sure about gamma ray bursts is that
they occur in the
> cosmic deep on an average of once a day, but never in the same
place twice, and
> appear to be thousands of times more powerful than ordinary
exploding stars,
> known as supernovas.
> But recent observations have led scientists to believe their
theories are on target
> in linking black holes to gamma ray bursts. They suspect there
may be more than
> one type of these bursts, but in all cases a relatively small
black hole is presumably
> the primary energy source.
> One idea involves the collapse of a massive star, many times
the heft of the Sun,
> into a rapidly spinning black hole, triggering the blast of
gamma radiation. Another
> possibility is the merger of two neutron stars, superdense
relics of exploded stars,
> which creates a black hole with explosive consequences. Or a
neutron star and a
> black hole, orbiting each other, could collide to create a
larger black hole and a
> similar cataclysm.
> Sir Martin Rees, a theoretical astrophysicist at Cambridge
University in England,
> said the evidence implicating black holes had strengthened
over the last two
> years, largely because astronomers had for the first time been
able to observe the
> immediate afterglow of some of these bursts in X-ray, radio
and visible
> wavelengths. The bursts last only a few seconds in gamma rays,
but leave more
> long-lived radiations in other wavelengths.
> "The challenge now is to fill in the detail," Dr. Rees said,
and spacecraft working in
> tandem with ground-based telescopes are making a promising
start, with improved
> observatories in the offing.
> In January, alerted by spacecraft detections of intense surges
of X-rays and
> gamma rays, automated telescopes on the ground swung into
action to get the
> first images in visible light of a gamma ray burst in
progress. Other observations
> soon followed in both visible and radio wavelengths. Studies
of the data have
> yielded new clues to the dynamics of the bursts and
confirmation that astronomers
> are not exaggerating when they describe them as the largest
explosions in the
> universe, second only to the Big Bang at the theorized birth
of the universe.
> Astronomers are now busy studying another especially bright
burst detected on
> May 10 by spacecraft and then observed in visual light at
ground telescopes,
> including the new European Southern Observatory telescope at
Cerro Paranal in
> Chile.
> In the April 27 issue of Proceedings of the National Academy
of Sciences, Dr.
> Dieter H. Hartmann of Clemson University in South Carolina
said research in
> gamma ray bursts "is currently in a supercharged phase, with
progress occurring
> rapidly."
> Although gamma rays are the most powerful form of
electromagnetic radiation,
> with energies far exceeding X-rays, those of cosmic origin
cannot penetrate Earth's
> atmosphere. Explosions of thermonuclear bombs, however, do
emit copious
> gamma rays into the atmosphere. The first cosmic gamma ray
bursts were thus
> discovered accidentally in 1968 by an American Vela
spacecraft, which was
> monitoring compliance with a treaty banning nuclear tests in
the upper
> atmosphere.
> The detections remained a cosmic oddity until 1991, when the
> Aeronautics and Space Administration launched the Compton
Gamma Ray
> Observatory. Its instruments established how common the bursts
are, recording
> some 350 of the transient surges of energy each year. They
seemed to be popping
> off everywhere, but they faded fast and could not be
correlated with any known
> celestial objects observed in other wavelengths -- no star or
galaxy, no stellar
> explosion or observed black-hole region.
> Then along came the Italian-Dutch BeppoSax satellite, launched
in 1996. It not
> only detected the original bursts of gamma rays, but also
associated flashes of
> X-rays. Because the X-rays are more focused, the satellite
could pinpoint their
> place of origin in the sky. Astronomers thus knew where to
look for any object in
> the region that could be the source of the blast.
> In this way, astronomers have been able to estimate distances
to these events,
> confirming that they occur well beyond the Milky Way, Earth's
home galaxy. Some
> are at least two-thirds of the way across the universe. Being
so luminous at such
> great distances confirmed that the gamma ray bursts indeed
bear messages from
> explosions of previously unimagined power.
> Astronomers estimated that the burst detected in January,
designated GRB990123,
> was 10 times as luminous as the brightest one seen before. At
first, some
> observers suspected that this might be an illusion. Perhaps
the radiations were
> being magnified beyond their true strength by the lensing
effects of intervening
> galaxies. At the symposium, scientists reported that further
analysis dispelled that
> idea.
> The distances also mean that these are events that occurred
very early in the
> universe, making them even more fascinating to scientists.
Their timing and nature,
> astrophysicists said, suggest that the bursts may be
associated with the periods
> of more prolific star formation in the universe's youth. Many
of the bursts have
> been detected in dusty star-formation environments.
> As probes of the early universe, Dr. Rees said, "the bursts
may be a way to use
> the universe as a laboratory to test fundamental physics,
short of the Big Bang
> itself."
> Taking a closer look at observations last year of a peculiar
and extremely powerful
> supernova, SN1998bw, astronomers noted that it appeared to
coincide in time and
> location with a gamma ray burst. The two could be different
manifestations of a
> single explosive event.
> "That got everyone excited," said Dr. Shrinivas Kulkarni of
the California Institute
> of Technology in Pasadena. It suggested that the collapse of
massive stars
> responsible for supernovas could in some cases form a black
hole and other
> conditions setting off a blast of gamma radiation as, in Dr.
Kulkarni's expression,
> "the dying cries of massive stars."
> Dr. Peter Garnavich, a supernova astronomer at the
Harvard-Smithsonian Center
> for Astrophysics in Cambridge, Mass., reported that the
supernova was at least 10
> times as energetic as others of its type and might not be a
rare phenomenon. Five
> such curiously powerful exploding stars have been studied in
the last two years.
> "The large energy required to understand" the possible
supernova-gamma ray
> burst pair, Dr. Hartmann of Clemson wrote, "is in contrast to
generic expectations
> for the explosion of such massive stars and suggest a new
class of stellar
> explosions: collapstars."
> Although scientists were reserving judgment on a connection
between some
> supernovas and gamma ray bursts, Dr. Mario Livio of the space
telescope institute
> said of the suspect SN1998bw: "Nature would have to be truly
malicious to put it
> in the same region of sky with a gamma ray burst for them not
to be related. More
> and more pieces of evidence indicate that these two phenomena
are related."
> The puzzle over these unusual supernovas goes to the heart of
the issue. As Dr.
> Kulkarni said: "The most interesting question is what forms
these things. What is
> the central engine? That is the big mystery of the gamma ray
> Scientists note two apparent differences in the gamma ray
bursts. Some erupt in
> only brief flashes, less than five seconds. These are detected
by the Compton
> Observatory, but do not last long enough to be picked up by
BeppoSax, which
> begins registering bursts only after five seconds. This is the
basis for the
> suspicion that there are different kinds of burst dynamics.
> Scientists think the longer bursts result from the
massive-star collapses related to
> extremely energetic supernovas, and the shorter ones stem from
> mergers or neutron star collision with black holes. The
result, in any case, is a
> black hole that sets in motion a rapid sequence of events.
> In what he calls the "best-buy model," Dr. Rees of Cambridge
pictured a rapidly
> spinning black hole with a mass of no more than three to five
times that of the Sun.
> (Supermassive black holes at the center of some galaxies, the
true ogres of the
> universe, are dense regions with estimated masses several
hundred million times
> the Sun's.) The black hole is surrounded by powerful magnetic
fields and a
> doughnut-shaped disk, or torus, of debris from the star
collapse or neutron star
> collision.
> Scientists at the symposium described what probably happens
next, in a matter of
> seconds. Much of the debris falls toward the black hole,
releasing enormous
> amounts of energy and matter. As an effect of the rotating
black hole and torus,
> the ejecta probably travels outward in a narrow jet
perpendicular to the torus. If
> this is true, and scientists say the circumstantial evidence
is strong, it would mean
> that a gamma ray burst is not equally bright in all
directions, but concentrates its
> energy in a beam. That, in turn, would mean that the total
energy output of the
> bursts is not as great as first thought, though still enough
to be nature's most
> powerful explosions.
> Internal shocks within the ejecta presumably trigger the
initial gamma ray burst. As
> the gamma rays encounter the interstellar medium, more shock
waves leave the
> lingering afterglow of X-rays, radio waves and visible light
shining as bright as a
> million normal galaxies. The afterglow can be studied for
hours and, in some cases,
> days.
> Scientists are looking to a pair of new spacecraft to provide
more comprehensive
> observations of the burst phenomenon and rigorous tests of
their theories.
> In January, NASA plans to launch the HETE, for Hyper-Energy
Transient Explorer,
> satellite to make rapid X-ray observations identifying the
location of each gamma
> burst. The agency is also weighing a proposal for a satellite
to be called Swift,
> which would be put in orbit in 2003. Its instruments would
make simultaneous
> observations in gamma ray, X-ray and visual wavelengths, with
greater sensitivity
> and frequency than current capabilities.
> It is by no means certain that, even then, astrophysicists
will be explaining gamma
> ray bursts with the same assurance as a Clarence speaking of
bells tolling for
> newly fledged angels.
> Copyright 1999 The New York Times Company
>Jay Respler
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